Method for heat-treating gallium arsenide monocrystals

ABSTRACT

A heat-treating method for an indium-doped dislocation-free gallium arsenide monocrystal having a low carbon concentration and grown in the Liquid Encapsulated Czochralski method, comprising a two-step heat treatment: 
     (i) heating the monocrystal at a temperature between 1050° C. and 1200° C. for a predetermined time length, and cooling the monocrystal quickly; and 
     (ii) heating the monocrystal at a temperature between 750° C. and 950° C. for a predetermined time length, and cooling the monocrystal quickly.

This application is a continuation; application of application Ser. No.07/327,491, filed Mar. 23, 1989, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a method for heat-treating indium-dopeddislocation-free gallium arsenide monocrystal with a view to improveproperties of the gallium arsenide monocrystals.

Gallium arsenide monocrystal is grown horizontally by the horizontalBridgman's method or vertically by the liquid encapsulated Czochralski(LEC) method. The monocrystal body obtained by means of the lattermethod is generally cylindrical except at the end portions (FIG. 4) sothat wherever the body is cut by a plane normal to the axis of the bodythe cross section of the body is circular. This is propitious forobtaining circular wafers, which are the industrial standard wafers andare, therefore, acceptable as they are to the device processing lineswhere the wafers are further processed. Also the liquid encapsulatedCzochralski method is regarded very highly for the facts that it iseasier to render a wafer dislocation-free in this method and that it isrelatively easy to obtain monocrystal gallium arsenide wafers ofrelatively large diameters in this method.

The gallium arsenide monocrystal is extensively used as a material tomake substrates for various devices such as light emitting diode, lightsensing device (i.e., light detector), devices for microwavecommunication system. Since the electron mobility is far greater ingallium arsenide monocrystal than it is in silicon semiconductor, thedevelopment of the substrates for integrated circuits having highelectron mobility has become very active recently. Therefore, to reducethe density of or to completely eliminate the existence of thedislocation in gallium arsenide monocrystal is an increasingly acutesubject of many researchers in this field of technology.

Incidentally, it is generally accepted that the stresses exerted on acrystal grown from the melt by a thermal shock is a chief cause ofdislocations in the crystals.

As a result of the efforts made with the view of obtainingdislocation-free gallium arsenide monocrystal, various techniques havebeen developed to render the gallium arsenide monocrystaldislocation-free, and among them are procedures specially applicable inthe liquid encapsulated Czochralski method. An example of the proceduresusefully adopted in the liquid encapsulated Czochralski method isdisclosed by B.C. Grabmaier et al., in Journal of Crystal Growth 13/14635-639 (1972). According to the disclosure, the increase in dislocationdensity caused by sealing or a seal crystal itself in the grown crystalespecially in the vicinity of the seed crystal/grown crystal interfaceis hindered by means of necking, which is a procedure comprising growinga 10 to 20 mm-long thin monocrystal portion having a diameter of 1 or 2mm from the seed, and adopting a small temperature gradient at the seedcrystal/grown crystal interface.

As a method for decreasing the dislocation density in gallium arsenidemonocrystal, a technique is proposed wherein additives such as Si, Te,Sb, Al, In, and B are added to the gallium arsenide monocrystal.According to the method, these additives have a tendency of adhering tothe dislocations as the dislocations develop in the growing crystalwhereby the propagation and growth of the dislocations are prevented.Among the additives, indium has proved a successful dopant forindustrially obtaining a dislocation-free gallium arsenide monocrystal.According to an example disclosed in Japanese Kokai No. 61-222991, it ispossible to obtain dislocation-free gallium arsenide monocrystal withoutapplying necking operation, if a molten gallium arsenide containingindium in an amount of about 6 weight % is prepared in a quartz crucibleto be employed as the melt, and the seed to raise the monocrystal fromthe melt is doped with indium in an amount of 0.7 weight %. Moregenerally, Japanese Kokai No. 61-222991 teaches that to obtaindislocation-free gallium arsenide monocrystal the seed should be dopedwith indium as much as kC weight % where k is a segregation coefficientof indium and C is the concentration of indium in the melt of galliumarsenide.

The indium-doped dislocation-free gallium arsenide monocrystal thusobtained is certainly free of dislocations, and from this fact itappears that the gallium arsenide is crystallographically uniform.However, it has been observed that thus grown indium-dopeddislocation-free gallium arsenide monocrystal rod, as it is, exhibitsnonuniformly in axial direction in such respects as electric properties,the concentrations of traps such as EL2, the concentration of carbonthat substituted arsenic, and occurrence of the micro precipitates. Evenif such a monocrystal is in semi-insulation state, the monocrystalsubstrates cut therefrom will be such that when they are made intotransistors by means of the ion-implantation procedure, the resultingtransistors have inconsistent threshold voltages Vth, which determinethe on-off positions in switching action.

An In-doped dislocation-free gallium arsenide monocrystal is ordinarilyobtained by the liquid encapsulated Czochralski (LEC) method. As shownin FIG. 3, the liquid encapsulated Czochralski method employs apressurized chamber (furnace) schematically drawn by the framedesignated by 1, and this pressurized chamber 1 is filled with an inertgas. A PBN (pyrolytic boron nitride) crucible 2 is installed in themiddle of the chamber 1 such that the crucible 2 is capable of beingrotated around its center line by means of a shaft. Reference numeral 3designates an In-doped melt of gallium arsenide contained in thecrucible 2. The surface of the melt 3 is entirely covered with a layerof another melt 4, which is of B₂ O₃ whereby the melt 3 is sealed fromthe gaseous environment. A heating means 5 is provided to controllablyheat the melt 3. The seed is lowered to the melt 4 and, penetrating thelayer of the melt 4, it is caused to touch the surface of the melt 3.Then, while the crucible is slowly rotated by the shaft in the directionindicated by a curved arrow below (FIG. 3), the seed is moved upwardvery slowly as it is rotated in the opposite direction as indicated by acurved arrow above. The monocrystal grows from the lower tip of the seedas the seed is raised, and a monocrystal ingot 10 as shown in FIG. 4 isobtained in the end. In FIG. 3, reference numeral 6 designates a heatshield.

The In-doped dislocation-free monocrystal ingot or rod 10 thus obtainedtends to have inconsistent properties along the axial direction, asstated above, and it is postulated that this is a result of the factthat the thermal history experienced by different portions of themonocrystal rod is not always the same. Especially the conditions underwhich the cooling process is conducted are difficult to maintain fixedor uniform in time with respect to the different portions of themonocrystal rod.

Koji Yamada et al reported in their preparatory treatise for the 47thAutumn Season Applied Physics Symposium, 22a-K-8 (1986), that as theyanalyzed an In-doped gallium arsenide monocrystal rod grown by the LECmethod by means of the infrared transmission method (IRT) as well as theKOH etching method, they observed the existence of microdefects whichwere not dislocation in the monocrystal rod. Further, they reported intheir preparatory treatise for the 34th Spring Season Applied PhysicsSymposium, 30a-Z-3 (1987), that the formation of the same microdefectswas strongly dependent upon the thermal history below 1000° C.

These reports showed that there exist microdefects which are notdislocations and are distributed nonuniformly, in the In-dopeddislocation-free gallium arsenide monocrystal rod and that the formationof the microdefects is affected by the thermal history during themonocrystal pulling-up operation. However, there has been disclosed nosolution for making the distribution of the microdefects uniformthroughout the monocrystal rod nor for eliminating the microdefects.

It is an object of the invention, therefore, to provide a method forthermally treating an indium-doped dislocation-free gallium arsenidemonocrystal rod which method is capable of making homogeneous in theaxial direction of the monocrystal rod the electric properties such asresistivity, the concentrations, i.e., the densities of deep traps suchas EL2, the concentration of carbon that substituted arsenic at arsenicsublattice site (C_(As)), and the density and size of microprecipitates,and thereby producing a gallium arsenide monocrystal from which it ispossible to obtain high quality gallium arsenide monocrystal substratesdesirous for manufacturing semiconductor devices.

SUMMARY OF THE INVENTION

The present invention, therefore, was contrived with the view of solvingabove-mentioned problems, and in particular, the invention provides aheat-treating method applicable to the In-doped dislocation-freemonocrystal manufactured in the LEC method during or, preferably, afterthe pulling-up operation. The heat treatment comprises a first and asecond heating steps which are conducted consecutively. The firstheating step adopts a heating temperature in the range from 1050° C. to1200° C., and the second step adopts a heating temperature in the rangefrom 750° C. to 950° C.

The first heating step has an effect of "initializing" the properties ofthe as-grown monocrystal ingot, which is to render the quality of themonocrystal ingot closely resemble that of the same monocrystal ingotimmediately after the crystal solidification at the temperature near themelting point. The second heating step is effective to stabilize theelectric properties of the monocrystal lest they should be affectedduring the thermal treatments in the later semiconductor devicemanufacturing processes. The second heating step is also effective toensure the electric semi-insulating state of the monocrystal (with aresistivity of 10⁷ Ωcm or higher). The lengths of time for the first andthe second heating steps are such that the heating operation iscontinued for at least half an hour from the moment the heatingtemperature reaches the respective predetermined values. It is alsorequired that the cooling operation following the first heating stepshould be conducted drastically at a cooling rate of from -10° to -200°C./minute. Similarly the cooling operation following the second heatingstep must be conducted such that the cooling rate is from -10° to -200°C./minute until the temperature becomes 300° C. or lower.

Described next will be the manners of inspecting the nonuniformity ofthe monocrystal ingot pulled from the melt, the effects of the first andthe second heating steps, and the effects of the controlled coolingrates.

Observation of ununiformity in an as-grown monocrystal ingot

An In-doped dislocation-free gallium arsenide monocrystal ingot (2"φ and200 mm long) was grown by the conventional pulling-up method and itcontained carbon in a concentration of less than 2×10¹⁵ cm⁻³. A samplepiece F was taken from the top (or front) portion (in the vicinity ofthe seed) of the monocrystal ingot, and a sample piece T was taken fromthe tail portion of the monocrystal ingot. Another In-dopeddislocation-free gallium arsenide monocrystal ingot was grown in amanner such that the pulling rate from the moment the seed was connectedwith the melt until the completion of the upper conical portion of theingot was at a conventionally adopted rate and that the solidifiedmonocrystal was disconnected from the melt by quickly pulling up themonocrystal at an increased rate of 20 mm/sec. A sample piece C wastaken from this monocrystal. The three samples F, T, and C were heatedat 950° C. for two hours, and then cooled at a cooling rate of 100°C./minute. The electric resistivity, the concentration of trap level EL2, the carbon concentration (C_(As)), and the density of the etching pitgiven rise to by the microprecipitate were measured before and after thetwo-hour-long heat treatment at 950° C. (The precipitation pit is thepit which is a recess etched selective at a microprecipitate on orbeneath the crystal surface.) The electric resistivity was measured bythe van der Pauw method. The EL2 concentration was measured by the nearinfrared absorption method employing a wave length of 1.1 μm. The carbonconcentration (C_(As)) was measured by the infrared absorption methodwith respect to the localized vibration mode in the vicinity of 580cm⁻¹. The precipitation (microdeposit) pit density was measured by theAbrahams-Buiocchi preferential etching (A-B etching) method.

Table 1 shows the results of the measurement of these items before thetwo-hour heat treatment at 950° C., and Table 2 shows the results of themeasurement after the rapid cooling following the two-hour heattreatment. FIG. 5 shows the change in the concentration of EL2 levelcaused by the two-hour heat treatment at 950° C. and the subsequentrapid cooling.

From the two tables, it is noted that no precipitation pit was observedin Samples T and C not only before but also after the two-hour heattreatment at 950° C. Also it is noteworthy that the concentrations ofEL2 of Samples T and C were low before the two-hour heat treatment, butthe heat treatment caused them to increase substantially. Also, as aresult of the heat treatment and the rapid cooling, these two Samples Tand C turned to be n-type semi-insulators. These phenomena are thoughtto be attributable to the fact that Samples T and C experienced a rapiddrop in temperature from 1000° C. or higher down to 300° C. during thecrystal pulling operation in the furnace. In contrast with this, as aresult of slow cooling during the pulling of the monocrystal ingot,microprecipitates were generated in the monocrystal in the case ofSample F which was taken from the vicinity of the seed of the ingotgrown in the conventional manner.

                  TABLE 1                                                         ______________________________________                                        Properties before          conical portion                                    2-hour heat treatment                                                                      conventional ingot                                                                          of ingot rapidly                                   at 950° C. and                                                                      Sample   Sample   disconnected from                              rapid cooling                                                                              F        T        melt, Sample C                                 ______________________________________                                        precipitation pit                                                                          .sup.˜ 1 × 10.sup.6                                                        0        0                                              density (cm.sup.-2)                                                           resistivity  0.2      8.0      0.6                                            (× 10.sup.7 Ω cm)                                                              (n-type) (n-type) (p-type)                                       [EL2] (× 10.sup.16 cm.sup.-3)                                                        1.0      0.4      <0.4                                           [C.sub.As ] (× 10.sup.15 cm.sup.-3)                                                  0.5      1.5      2.1                                            ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Properties after           conical portion                                    2-hour heat treatment                                                                      conventional ingot                                                                          of ingot rapidly                                   at 950° C. and                                                                      Sample   Sample   disconnected from                              rapid cooling                                                                              F        T        melt, Sample C                                 ______________________________________                                        precipitation pit                                                                          .sup.˜ 1 × 10.sup.6                                                        0        0                                              density (cm.sup.-2)                                                           resistivity  6.5      3.3      3.1                                            (× 10.sup.7 Ω cm)                                                              (n-type) (n-type) (n-type)                                       [EL2] (× 10.sup.16 cm.sup.-3)                                                        1.0      1.2      1.7                                            [C.sub.As ] (× 10.sup.15 cm.sup.-3)                                                  0.5      1.5      2.1                                            ______________________________________                                    

The effect of the first-step heat treatment

Sample F obtained from the vicinity of the seed of the monocrystalpulled in the conventional manner and containing microprecipitates in itwas heated at temperatures higher than 1050° C. for a period longer thanhalf an hour. Preferably, this heat treatment should be conducted attemperatures higher than 1100° C. for a period longer than two hours.Then, Sample F was rapidly cooled at cooling rates of from 10° C./minuteto 200° C./minute, whereupon the density of the precipitation pitsbecame zero. This phenomenon can be explained by the assumption that themicroprecipitates formed in the crystal matrix in the incidental coolingprocess during the conventional monocrystal pulling-up operation weredecomposed by being heated to the high temperature near the meltingpoints of the solidified materials, and thereby the sizes of themicroprecipitates became extremely small. These microprecipitates areknown to comprise excess arsenic, according to appropriate methods ofanalysis, and these precipitates are thought to be formed in thecrystallographic lattice matrix of the gallium arsenide monocrystalduring the crystal pulling operation.

The effect of the second-step heat treatment

It was found from the experiment described above that in order to turnthe entire body of a monocrystal ingot obtained in the conventionalmanner to be free of microprecipitates, the monocrystal ingot should beheated at temperatures higher than 1050° C. for 30 minutes or longer, orpreferably at temperatures higher than 1100° C. for two hours or longer,to thereby decompose the microprecipitates, and, thereafter, themonocrystal ingot should be cooled rapidly so as not to allow recurrenceof the microprecipitation. However, as Tables 1 and 2 and FIG. 5indicate, the gallium arsenide monocrystal obtained through the heattreatment at a temperature near the melting point (≧1050° C.) and thesubsequent rapid cooling is so unstable in its resistivity andconcentration of EL2 when subjected to a heat treatment in thetemperature range from 750° C. to 950° C. that it cannot be said to be astable n-type semi-insulator. And a monocrystal ingot must be thermallystable since it is subjected to heat treatments in the temperature rangefrom 750° C. to 950° C. in the subsequent semiconductor devicemanufacturing processes. To avoid changes in electrical properties suchas resistivity during the later heat treatments in the semiconductordevice manufacturing processes, the monocrystal needs to be subjected toanother heat treatment after the first-step heat treatment. In order tostabilize the monocrystal against the later heat treatments, thissecond-step heat treatment should adopt temperatures in the range from750° C. to 950° C. which are equivalent to the temperatures experiencedby the monocrystal during the heat treatments in the semiconductordevice manufacturing process. The length of the second-step heattreatment should be 30 minutes or longer, and after this the monocrystalshould be forcibly cooled.

After reducing the density of the precipitation pits to zero byconducting the first-step heat treatment at temperatures higher than1050° C., the monocrystal of gallium arsenide was subjected to thesecond-step heat treatment. Thus-treated monocrystal was examined and itwas found that the difference between the concentration of EL2 in thevicinity of the seed and that in the tail portion was much smaller thanthe difference was in the case of the ingot of Table 2. It was alsoobserved that the electric resistivity was almost perfectly uniformthroughout the ingot. The inventors came to believe from the results inTable 2 that the small difference between the concentration of EL2 inthe vicinity of the seed (Sample F) and that in the tail portion (SampleT) is essentially attributable to the nonuniform distribution of themicroprecipitates in the monocrystal ingot with respect to the crystalgrowth direction.

The effect of cooling rate

It is necessary to adopt a cooling rate of 10-200° C./minute in thecooling operation conducted between the first-step heat treatment andthe second-step heat treatment in order to prevent recurrence of themicroprecipitation. If a cooling rate of 0.1° C./min is adopted, forexample, the microprecipitation is inevitable. The second-step heattreatment and the pursuant rapid cooling are essential so as to obtain asemi-insulator having an electric resistivity of 10⁷ Ωcm or higher, andif the cooling rate is as low as 0.1° C./minute, for example, thesemi-insulating property is largely lost, even though the materialbecomes n-type.

As is described above, a monocrystal ingot grown in the conventionalmanner tends to have an unfavorable characteristic, which is that itsvarious properties are not uniform and consistent throughout its bodydue to the uneven thermal history it inevitably experiences. However, bysubjecting such an ingot to an appropriate heat treatment conducted attemperatures in the vicinity of the melting point of the ingot, bycooling the ingot at an expedited rate, and then by once more subjectingthe ingot to a second-step heat treatment followed by another quickcooling, it is possible to improve the ingot such that the ingotdisplays no nonuniformity or inconsistency in the important propertiessuch as resistivity, in the direction of the axis of the ingot growth,and also thus-improved monocrystal ingot retains good semi-insulatingproperty, which is a property essential to the precision manufacturingof semiconductor devices.

The heat treatments can be effectively applied to a gallium arsenidemonocrystal ingot as it is or after the ingot is cut into slices, i.e.,substrates. Preferably, the heat treatments are applied to the slicedmonocrystal substrates all at a time, since it is easier to effectuniform heating with the substrates than with the thick ingot, and alsothe substrates can be cooled quickly and uniformly, whereas it takeslonger time to cool the ingot because the inside of the ingot is lessresponsive to the cooling temperatures. Therefore, the quenching effectof rapid cooling is greater with the sliced substrates than with theingot in the same cooling procedure, and furthermore in the case of thesubstrates, a bolder choice of cooling rate is possible. The upper limitfor the cooling rate is determined based on the considerations ofpreventing distortion and destruction of the monocrystal rod orsubstrate due to the thermal shock during the quick cooling operation,and also of preventing degradation of crystallinity due to slipping.Special care must be taken during the cooling operation after thefirst-step heat treatment when the temperature has been raised to thelevel of the melting point of the monocrystal, lest the gallium arsenidemonocrystal rod or substrate should be distorted and thereby undergodegradation in crystallinity and destruction.

It is also possible to obtain the effect of the invention, when themonocrystal is cooled to room temperature after the first-step heattreatment, and then the monocrystal is heated again to go through thesecond-step heat treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram representing the temperature vs. time controlprogram in accordance with the heat treatment method of the invention;

FIG. 2 shows a graph wherein the resistivity of the gallium arsenidemonocrystal rod is plotted with respect to the location in the axialdirection of the rod.

FIG. 3 is a vertical sectional view depicting a crystal pullingapparatus adapted for liquid encapsulated Czochralski (LEC) method;

FIG. 4 is a perspective view of a gallium arsenide monocrystal ingot;and

FIG. 5 is a chart showing the changes in the concentration of EL2 in thetwo gallium arsenide monocrystal rods, one being grown in theconventional manner and the other in a manner such that the crystal coneis quickly disconnected from the melt, before and after the heattreatment of 950° C. for two hours with rapid cooling is applied.

DESCRIPTION OF THE PREFERRED EXAMPLE

Referring to the attached drawings, an example of the invention shall bedescribed hereinbelow.

In a preferred example of the invention, a gallium arsenide monocrystalrod 10 grown by the LEC method in a crystal pulling apparatus shown inFIG. 3 was sliced into thin circular plates. The first-step heattreatment was applied to the monocrystal plates by a heater. Referringto FIG. 1, the heating temperature applied to the monocrystal plates wasraised from room temperature T₀ to a predetermined temperature T₁, whichwas 1100° C. in this example. The temperature was maintained at 1100° C.for a predetermined time length t₁, which was five hours. Then thetemperature was caused to drop to a predetermined temperature T₂ (950°C.) at a cooling rate of about 100° C./min.

Thereafter, the gallium arsenide monocrystal was subjected to thesecond-step heat treatment which comprises heating the gallium arsenidemonocrystal at T₂ (950° C.) for a predetermined time length t₂ (twohours in this example). Then, the temperature was again caused to dropsharply at a cooling rate of about 100° C./min until it reaches 300° C.

In FIG. 2, the curve a represents the profile of electric resistivity ρin the axial direction of the monocrystal as-grown ingot taken after theingot was pulled up from the melt; the curve b represents the profile ofelectric resistivity ρ in the axial direction of the monocrystal ingottaken after the ingot was pulled up and subjected to the second-stepheat treatment only (i.e., no application of the first-step heattreatment); and the curve c represents the profile of electricresistivity ρ in the axial direction of the monocrystal ingot takenafter the ingot was pulled up and subjected to both the first- andsecond-step heat treatments. As is apparent from FIG. 2, by virtue ofthe two-step heat treatment of the invention, the monocrystal ingot 10obtained highly even resistivity profile along the axis of the ingotgrowth. Although FIG. 2 shows only the profiles of electric resistivityρ, other properties such as the concentration of EL2 and the density ofmicroprecipitates were found to be highly consistent along the growthaxis of the monocrystal ingot. Especially, the precipitation pit densitywas virtually zero throughout the length of the ingot.

Therefore, the method of the invention was found to be effective inleveling the variation in crystal property along the axis of themonocrystal ingot, and thus effective to substantially eliminate thedifference between the values of various properties at one end of theingot and those at the other end. As a result, the integrated circuits(IC) manufactured on the gallium arsenide monocrystal substrate of theinvention can attain very high performance and precision.

According to the invention, therefore, the first-step heat treatmentinitializes the quality of the gallium arsenide monocrystal ingot, thatis, the treatment renders the quality of the monocrystal ingot almost asit was immediately after the ingot was solidified after being pulled upfrom the melt. The subsequent second-step heat treatment stabilizes theelectric properties of the ingot and imparts semi-insulating property toit. As a result of the combined effect of the first-step and thesecond-step heat treatments, the microprecipitation pit density becomeszero throughout the monocrystal ingot and thus it is possible to obtaina gallium arsenide monocrystal of excellent and uniform crystal propertyalong the axis of crystal growth. The invention therefore enablesmanufacture of semi-conductor chips having high and accurate electricperformance.

What is claimed is:
 1. A heat-treating method for producing asemiconducting indium-doped dislocation-free gallium arsenidemonocrystal having a resistivity of 10⁷ Ωcm or greater, saidheat-treating method comprising conducting the following steps after amonocrystal having a low carbon concentration of 2×10¹⁵ cm⁻³ or lower isgrown in a liquid encapsulated Czochralski method:(i) heating themonocrystal to a temperature between 1050° C. and 1200° C. for at least30 minutes; (ii) cooling the monocrystal at a cooling rate of from 10°C. per minute to 200° C. per minute; (iii) heating the monocrystal at atemperature between 750° C. and 950° C. for at least 30 minutes; and(iv) cooling the monocrystal until the temperature reaches 300° C. orlower at a cooling rate of from 10° C. per minute to 200° C. per minute.